The separation angle between the illumination beam and the reflected ON-beam is a limiting factor for the optics and the geometric design of MOEMS displays. Often the modulated beams are reflected in a direction which is normal (perpendicular) to the plane of the mirror array of the MOEMS. The normal direction leads to a simple layout of beam geometry for both the split of the light beam which illuminates the image modulators and, even more so, for the preferred orthogonal and normal superposition. Single and Multi-channel displays with micro opto electro mechanical systems (MOEMS) as image modulators according to the state of the art use TIR prisms to separate the illumination beam and the normally reflected modulated ON-beams. A non-normal direction of the reflected modulated beam is found in the state of the art in single MOEMS displays without superposition, and without the essential requirement of a splitting system. Here, often a TIR-prism for separation is not necessary.
With a non-normal direction of the reflected beam, however, a increased angle between the incident illumination beam and the modulated ON-beam can be achieved, which results with an improved separation of these beams. A multi-channel display with MOEMS as image modulators is therefore desired, in which a superposition system superposes modulated image beams, which are reflected from the mirror arrays in a non-normal direction, to reduce the complexity which is necessary in multi-channel displays to separate illumination and ON-beams with normally reflected ON-beams. In contrast to the often orthogonal and normal superposition seen in the prior art, which in many cases can be illustrated comparatively easily in two-dimensional representations of their arrangements, the spatial complexity resulting from the non-normal direction of the ON-beams is greatly increased. Our aim was to find a solution for the requirements put by this spatial complexity.
MOEMS Geometry
To understand the present invention, it is important to know the geometry of the MOEMS (in earlier literature often without the “O”). These MOEMS comprise an array of micro mirrors, which are deflectable (“tiltable”) around a mirror deflection axis. In many current Multi-MOEMS displays a MOEMS by Texas Instruments, the so-called DMD (digital mirror device) is used (e.g. U.S. Pat. No. 5,535,047). The geometry of illumination beam, reflected image beam (ON-beam) and the OFF-beam, reflected in a direction different from the ON-beam, is shown in FIG. 10a,b. 
The DMD comprises an array of quadratic micro mirrors, with a side length below 20 μm, which are deflectable around a deflection axis which is arranged at the diagonal of the mirrors. In FIG. 10a,b the individual mirrors are omitted to simplify the drawing. The plane of the whole mirror array is indicated by the circle (100). The normal (101) to this plane is indicated by the dashed line. The DMD is a bistable system: the single mirrors are stable in an ON- and in an OFF-state, while the mid-position between ON- and OFF-state, parallel to the plane (100) of the array, can not be addressed or stabilized. Each micro mirror can be deflected by the deflection angle β, (which currently is 12°) from the instable mid-position to the ON-state, which is described in the drawing by the normal (102) of the single mirror surface in the ON-state. The OFF-state is the second stable state, and it is characterized by the same deflection angle β, but opposite to the side of the ON-deflection. The OFF-state is described by the normal (103) of the single mirror surface in the OFF-state. An illumination beam, incident on the array, can be reflected as ON-beam (12) into a direction to be displayed, or can be reflected as OFF-beam (13) into a direction to be disposed. In Multi channel displays with MOEMS according to the state of art (e.g. DE10361915, U.S. Pat. No. 7,403,320, U.S. Pat. No. 7,466,473, U.S. Pat. No. 7,817,329, U.S. Pat. No. 7,134,416, U.S. Pat. No. 7,375,896, US2009/0027624, US2009/0027624 [col. 3, lines 60-62], U.S. Pat. No. 5,535,047) the illumination is chosen such that the modulated ON-beam (12) is reflected in a direction normal to the plane (100) of the micro mirror array, which is given by the normal (101) in FIG. 10a. This normal direction of the modulated beam (12) is a result of the illumination beam (11) being incident in the plane defined by the normals (102, 103) of the mirrors in the ON- and OFF-states with an exact angle of β to the normal (102) of the mirror in the ON-state (and 2*β to the normal (101) of the array) For the understanding of our disclosure it is important to see that all beams (11, 12, 13), and all three normals (101, 102, 103) are in a common plane. This is the most simple spatial geometric arrangement.
In displays with more than one MOEMS modulator the direction of the mirror deflection axis, which includes an angle of 45° to the rows or columns and is the diagonal of the mirrors, becomes an additional burden. All arrays produced so far have a rectangular, non-quadratic shape, with an array width (corresponding to the image width) being larger than the array height (corresponding to image height). This makes the DMD, when input and output beams are taken into account, a stereo-isomer (it shows “handedness”). Thus, for the superposition of DMDs a folding additional to the folding of one channel at the superposition layer is required, because there has to be either the same number of foldings in each channel or an even-numbered difference of the number of foldings (U.S. Pat. No. 5,638,142, U.S. Pat. No. 6,250,763B1). Only by using stereo-isomeric pairing, internally symmetric MOEMS, or in very special arrangements an uneven number of foldings in the two channels is possible (DE 10361915). Solutions of the state of the art which meet these requirements include complex arrangements, e.g. an assembly made of several prisms (Tri-chroic Prism Assembly, TPA, U.S. Pat. No. 7,396,132, US2007/0229770), or particular arrangements of the MOEMS with selectively modeled TIR-prisms, as described in U.S. Pat. No. 7,375,896.
The separation of the incident beam and the modulated beam in Multi-MOEMS displays according to the state of the art is shown in FIG. 10b. 
Because the angle between the illumination beam incident on the micro mirror array and the reflected modulated ON-beam is only 2*β, in all Multi-MOEMS displays of the state of the art at least one TIR-prism (17) is used to separate these beams. FIG. 10b shows a situation where a total internal reflection layer folds the incident beam (11) and transmits the modulated beam (12). In some arrangements (not shown) the TIR-prism is used in an inverted way (comp to FIG. 10b), whereby the incident beam transmits the TIR-layer and the modulated beam is reflected (US2002/0021505, U.S. Pat. No. 7,360,905). Although the angles of the relative TIR-layers are chosen for the specific requirements, the normal direction of the modulated beam is used throughout.
While recent Multi-MOEMS displays cannot do without beam separation systems, in the state of the art there are single MOEMS displays, which get along without these separation systems, because they use a different illumination beam arrangement (e.g. US2007/0247591, U.S. Pat. No. 6,540,361). The application of a DMD, in which the modulated beam leaves the modulator in a non-normal direction provides a larger angular distance between the axis of the illuminating and the modulated beams. This application is shown in FIG. 10c. 
Similar to FIG. 10a, the incident, the modulated ON- and the OFF-beam, as well as the normals of the mirror array (101) and the normals (102, 103) of the planes of single mirrors in their ON- and OFF-state are shown. Again, the geometry of a recent DMD and thus the same angles for ON- and OFF deflections are used as in FIG. 10a. Here, they are described as ON-deflection angle β1 and OFF-deflection angle β2, because MOEMS have been disclosed in the state of the art which have different ON- and OFF-deflection angles, and because only the ON-deflection angle β1 becomes relevant for the disclosure of our invention. The ON-beam (12) includes an angle α with the normal of the normal (101) of the plane (100) of the micro mirror array. In this art, a mid-sagittal direction is preferred for the modulated ON-beam (12). A sagittal plane can be thought to be spanned by a vertical line through the array—(parallel to the columns of the array, not shown) and by the normal (101) of the plane of the array, the mid-sagittal plane would accordingly be spanned by a vertical line in the center of the array and the normal (101). This leads to several geometric consequences for the illumination beam and for the OFF-beam. First, to achieve the preferred direction of the reflected ON beam, the illumination beam (11) has to be directed onto the array in a plane, which is not identical with the plane spanned by the normals (101, 102, and 103). This is indicated by the different lines connecting the three normals (101, 102, 103) and the lines from the illumination beam (11) to the modulated ON-beam (12). The choice of the angles and planes to be used for illumination (and also for the disposal of the OFF-beam) requires much more thought than in FIG. 10a. The plane for the input beam is determined by the axis of the modulated beam (12) and the normal (102) of the surface of the single mirrors in the ON-state. The angle included between the normal of the incident beam (11) and the normal (102) can not be expressed as a linear combination of the angles α and β1, because they are not in the same plane. We therefore introduce the illumination angle δ, which directly specifies the angle between the incident illumination beam (11) and the normal (102) of the mirror surface in the ON-state. A fourth angle γ could be used to specify the angle between the incident beam (11) and the normal (101) of the array. Here we only use it to emphasize that γ is not the sum of δ and β1. Likewise, δ is not the sum of α and β1, but smaller. If the requirement of a mid-sagittal ON-direction in this prior art for single MOEMS displays is released, it becomes evident that the relations between δ, α and β1 remain open and δ could even be chosen to be smaller than β1. For the disclosure of our invention it is to be noted here already that the angle δ can exceed the ON-deflection β1. Because the incident and the reflected beams include an angle of 2δ, the separation of incident and reflected ON-beams can be significantly enhanced above the 2β (resp. 2β1) condition with a normally reflected ON-beam.
Not described in the state of the art is the illumination scheme shown in FIG. 10d. Although this is a special case of the general concept explained in FIG. 10c, it is relevant for some embodiments of the disclosure, and will be referred to when these are described.
In contrast to FIG. 10c, the illumination geometry has been changed to show a special choice of incidence, where all beam axes and all normals are in a common plane, quite similar to the geometry shown in FIG. 10a. This includes the incident beam (11), the normals (102, 103) of the mirror in ON- and OFF-state, the normal (101) of the plane (100) of the micro mirror array, the ON-beam (12) and also the OFF-beam (13) used to dispose light at dark image points. This is an substantial simplification of the general situation which facilitates the design requirements especially for Multi-MOEMS displays, where there has to be a clean separation of all those beams. The common plane is indicated by the sector circle line spanning from illumination beam (11) to the OFF-beam (13). The common plane has the following consequence: in FIG. 10d, the illumination angle (δ) consists of the ON-deflection angle (β1) plus the zenith angle (α). The incident illumination beam (11) is separated by an angle of 2*δ from the ON-beam (12), a separation which is exactly 2*(β1+α), while in a less than optimal geometry (comp. FIG. 10d) this distance is smaller, and in fact, could be even smaller than 2*β1, which is the separation in normally reflected ON-beams in Multi-MOEMS displays according to the state of the art. The optimized angle separation could, as an example, easily be realized with a new MOEMS architecture according to the state of the art. While recent DMDs by Texas Instruments have a diagonal deflection axis, MOEMS disclosed by Fraunhofer comprise mirrors, which are tiltable around an deflection axis orthogonal to the mirror. Depending on the position of the modulator and the image, these mirrors rotate either from left to right, or from top to bottom (and vice versa). Optimized angle separation can, however, also be realized with the DMD type of MOEMS.
Multi-MOEMS displays with this type of illumination are disclosed by this paper, but our invention is not limited by this type of illumination. The invention however depends on a illumination which improves the discrimination of IN- and ON beams as compared to that described in detail in multi-MOEMS displays with normally reflected ON-beams according to the state of the art. These two illumination conditions of the prior art and of the disclosed display is compared in FIG. 10e. 
FIG. 10e illustrates the consequence of the illumination system with an increased separation angle between the illumination beam and the reflected modulated beam, which is used in our invention of Multi-MOEMS displays (11L, 12L) and compares it to the illumination in Multi-MOEMS displays according to the state of the art (11S, 12S). Angle separation has a direct consequence on the F-number of the system. The F-number of projective devices has been described by the equation F1 (F-number of the illumination light path)=F2 (F-number of projection light path)=1/(2(2*NA)), and the numerical aperture (NA) being given by the sine of the (ON-) deflection angle. This results, when using a recent DMD with a deflection angle of 12° with an F-number of appr. 2.4. While the consequence of the deflection angle on the F-number is common knowledge among those known in the art, the effects of optimizing the distance between ON- and IN beams with an alternative illumination path has not been realized in projection devices with more than one spatial light modulator. As an example, provided with our figures, in the illumination system shown in FIG. 10d,e, an assumed azimuth angle (α) of 5° and a resulting illumination angle (δ) of 17° (in a state of the art DMD system with 12° deflection) leads to an F-number of 1.7, which is a significant improvement. The smaller circles indicate the maximum illumination cones possible on the illumination beam (11S) in prior art compared to the much larger circles which indicate the maximum illumination cones in the optimized separation angle illumination (11L). There are several limitations, e.g. the direction of the ON-beam (13L), to increasing the separation angles, which however are not relevant to the core of the invention and will therefore not be discussed here. Alternatively to improving the numerical aperture, the larger separation angle might render a TIR-prism for supporting the separation unnecessary.
Multi-MOEMS Display Arrangements: Layout in the State of the Art.
FIGS. 11 and 12 exemplarily show a 2-MOEMS and a 3-MOEMS display according to the state of the art with at least one TIR-prism to separate the illumination from the modulated beam and with normally reflected ON-beams. In FIG. 11 a schematical illustration of a 2-MOEMS display, a planar arrangement of two TIR prisms (17, 27) a split layer (74) and a superposition layer (14), is shown. In this 2-channel display system with two MOEMS (1,2) split and superposition are orthogonal and normal. Although the drawing is reduced to a top view, it is evident that the illumination beams (11, 21) are guided to the mirror arrays such that the modulated ON-beams (12, 22) are reflected in the direction of the normals (101, 201) of the planes of the two mirror arrays. The TIR-prism, used to reflect the illumination beams onto the MOEMS, transmit the ON-beams towards the superposition layer (14) and thus supports the beam separation with this comparatively small separation angle of normal ON-beam reflection. A normal superposition at (14) superposes the two ON-beams, and a superposition beam (4) of the two channel specific sub-beams (15, 25) leaves the arrangement to be displayed.
FIG. 12 shows schematically a widely-used trichroic prism assembly in a side view (FIG. 12a) and a topview (FIG. 12b). Here, only the main characteristics relating to our invention are described. FIG. 12a shows a common input beam (71), which is reflected into the assembly by an input TIR-prism (17). Two dichroic layers (14, 24), located at the back of two triangular prisms, split a read and a blue beam as illumination beams for the MOEMS (2,3), while a green illumination beam (11) transmits both split layer and becomes incident on the MOEMS (1). For a better readability, the illumination beams (21, 31) for the MOEMS (2,3) are omitted in the drawing, the first illumination beam (11) can be seen in FIG. 12a. The illumination beams are guided such onto the mirror arrays that the modulated ON-beams (12, 22, 32) are reflected normally from the plane of the mirror arrays, parallel to the normals (101, 201, 301) of the planes of these arrays. The modulated beams (22, 32) are folded two times, first by an unspecific total reflection, and than by the same dichroic layers (14, 24). These layers, formerly used to split the illumination beam, are now used to superpose the modulated beams. The green ON-beam (12) transmits all these reflection layers. Both superposition steps, the superposition of the first and second ON-beams (12, 22) at the superposition layer (14) and the superposition of the third ON-beam (32) with the first two at the superposition layer (24) the are not orthogonal, but are normal. The superposed beam (4, consisting of 3 sub-beams) finally transmits the input TIR-prism (17).